The present invention relates to a focusing means for a symbol code reader, such as a bar code reader, comprising a lens system, as well as to a bar code reader comprising such a focusing means. Such a focusing means, as well as such a symbol code reader, are known in practice.
Both active and passive readers can be used for reading bar codes or similar symbol codes. In an active reader a thin beam generated by a laser is used to scan the code and the light scattered from it is detected to obtain the information contained in the code. To form such a thin beam focusing optics are often used. In a passive reader the code is imaged onto a sensor array, such as a CCD-array, the array producing signals representative of the bar code. In such a passive reader, ambient light with or without additional illumination is used.
In an active reader the size of the beam in the point of incidence on the bar code should be small, such that the smallest bar width can be detected. In the passive reader the resolution of the imaging optics should be sufficient to detect the smallest bar width by means of the sensor array. In addition, the light scattered from the bar code should be collected effectively so as to enable the sensor array to produce reliable image signals. For these reasons, the aperture of the optical system used in the reader should be large. Such a large aperture, however, leads to a small depth of field. For practical reasons the range of distances at which a bar code can successfully be read, should be as large as possible. Obviously this requires a large depth of field.
In order to obtain a high light collection efficiency and a high spatial resolution as well as a large depth of field, the bar code reader should have a variable focusing distance. In practice the relative positions of the light source (laser) and the focusing optics in active scanners, as well as of the imaging optics and the sensor array in passive scanners, are preferably fixed while varying the focusing distance. This can be achieved if the bar code reader is provided with focusing means having a variable focal length.
EP-A-0,276,589 discloses a bar code symbol reader for changing the working distance of a scanning beam. FIGS. 1-5 disclose embodiments of bar code readers by which the effect of changing working distance of the scanning beam is obtained by a complex system of mirrors and lenses. In an arrangement shown in FIGS. 6A and 6B a changing working distance is obtained by using a light-transmissive rotary plate having different plate portions with different optical distance characteristics. By using a focusing lens with a high magnification factor downstream of the plate a small difference in the thicknesses of the plate portions can cause a large and major shift in the position of the focused beam spot. The arrangement shown in FIGS. 6A and 6B has as many beam spot positions as there are portions on the rotary plate. In FIGS. 7 and 8 a component is shown that, when used instead of the rotary plate in the arrangement of FIGS. 6A, 6B, provides a continuous shifting focused beam spot. The focal length is not changed. Furthermore, only refractive components are used.